1. Field of the Invention
The present invention relates in general to a process for the selective hydrogenation of acetylene present in an ethylene product stream, and more particularly pertains to a control system for such a process which optimizes the conversion of acetylene to ethylene with a minimum consumption of hydrogen. As used herein, the term "acetylene" includes the hydrocarbon C.sub.2 H.sub.2 as well as other acetylenic hydrocarbons, and the term "ethylene product stream" includes streams containing the hydrocarbon C.sub.2 H.sub.4 as well as streams containing other mono- and diolefinically unsaturated hydrocarbons.
It would be desirable to have a system and process for the accurate and controlled hydrogenation of acetylene in an ethylene product stream for both economic and operational benefits including, provision of a more consistent product quality, reduction in the amount of ethylene hydrogenated to ethane in the acetylene reactor, elimination of ethylene production loss due to acetylene reactor shut-down required by process upsets, extension of the life of catalysts by elimination of reactor runaways, and reduction of the amount of time an operator must spend attending to achievement of a tighter control over the process, increase in run time between regeneration of catalyst by reduced formation of heavy hydrocarbon poisons, and reduction of hydrogen consumption.
2. Description of the Prior Art
The catalyzed selective hydrogenation of acetylene to ethylene is a well known industrial process (see for example U.S. Pat. Nos. 2,802,889; 2,814,653; and 3,113,980). Ethylene product streams are usually contaminated with small amounts of acetylene which must be removed at least to the level of a few parts per million for the stream to meet process requirements, for example, in polymerization processes or to avoid formation of explosive acetylides in equipment. The conversion of minor concentrations of acetylene to ethylene in ethylene product streams is conventionally carried out by hydrogenating the acetylene in the presence of a suitable hydrogenation catalyst. Ideally, substantially all of the acetylene should be hydrogenated to ethylene without resulting in any appreciable hydrogenation of ethylene to ethane and/or ethylene polymerization to form "green-oil", and an excessive consumption of hydrogen. In practice this has been difficult to accomplish. Generally, ethylene plant acetylene converters are operated with a view toward maintaining an overconversion reaction to prevent acetylene "leakage" as this reduces the likelihood of producing off-specification ethylene. However, over-conversion has the disadvantage of increasing the hydrogenation of ethylene to ethane thereby reducing the recovery of ethylene and increasing the consumption of hydrogen. Polymerization of ethylene results in further loss of ethylene and produces a "green-oil" type hydrocarbon material that accelerates deactivation of the catalyst. Attempts have therefore been made to optimize or precisely control the selective hydrogenation of acetylene present in ethylene streams to maximize ethylene recovery and minimize hydrogen consumption.
U.S. Pat. No. 3,471,582 describes a control system for the selective hydrogenation of acetylenic components of an olefin/diolefin stream in a series of hydrogenation reactors which is based upon control of hydrogenation reaction temperature. In accordance with this control system, the temperature within the hydrogenation reaction zone is controlled by manipulating the reactant feed temperature between the product and reactant streams to thereby maintain a predetermined difference in temperature. When the product temperature reaches a predetermined maximum, control of the reactant feed temperature is automatically switched to be responsive solely to the product stream temperature. Control in this manner continues until the product temperature falls below the predetermined maximum, at which time control of the reactant feed temperature is automatically switched to be responsive to the difference in temperature between the product and reactant streams. Similarly, U.S. Pat. No. 3,656,911 provides a selective hydrogenation control system for a series of hydrogenation reactors in which the temperatures of the feed streams introduced into the reactors are controlled in response to temperature and flow measurements and analyses of the feed and effluent streams. U.S. Pat. No. 3,839,483 describes a control system for a single hydrogenation reactor in which the ratio of hydrogen to acetylene in the feed to the reactor is adjusted in response to the concentration of acetylene and hydrogen in the reactor effluent, and the temperature of the feed to the reactor is adjusted responsive to the hydrogen concentration in the reactor effluent.